High voltage transformers contain paper insulated conductors immersed in an oil-filled tank. During the manufacturing process, the moisture content of the paper insulation is reduced to less than 0.5 percent. However, as a transformer ages, the moisture content of the paper may gradually increase as moisture slowly diffuses from the atmosphere into the oil, and then into the paper insulation. Excessive moisture content in paper insulation quickly deteriorates its insulating ability, reducing the electrical resistance of the insulation and promoting local heating and surface tracking under operating electrical stress.
Moreover, under thermal stress as experienced either during normal operation or during emergency overloading of a transformer, excess water molecules may be converted into free gas bubbles, which under electrical stress will produce partial discharges that gradually degrade the oil-paper insulation. Free water molecules are generated when the paper insulation degrades, resulting in a vicious cycle: an increase in the moisture content of the paper above about 2 percent can lead to additional local overheating, gassing, increased partial discharge activity, more water generation, and eventually run away loss in the integrity of the insulation system resulting in a premature failure of the equipment.
During manufacturing, a dissipation factor at 60 Hz of the overall insulation system, either between the primary and secondary windings of the transformer or between a winding and the tank, is used to determine the "dryness" of insulation. For transformers in service, the moisture content in the paper insulation is estimated through measurement of the water content of the transformer oil. However, in both cases the moisture content of the paper insulation cannot be known exactly. Until the present invention there were no practical methods available for direct on-site measurement of moisture content of the paper insulation in transformers.
The present invention provides a method and device for directly measuring the moisture content of paper insulation in electrical transformers. The invention can be used to control "dryness" of the paper insulation in the manufacturing process, resulting in a better product. Furthermore, use of the subject invention can reduce maintenance costs through prevention of unnecessary treatment of transformer insulation, can improve the reliability of power transformers, and can permit full use of the normal capacity and emergency overloading capability of transformers without excessive aging. The method and device of the subject invention can be used to directly measure the moisture content of any absorbent material, the paper insulation in a transformer being an illustrative example.
The present invention utilizes an infrared absorption technique to measure moisture content. A water molecule has many absorption bands in the infrared region of the electromagnetic spectrum. Absorption in any of these bands can be used to determine the moisture content in a sample. For example, water exhibits almost no absorption of radiation at 1.81 .mu.m and pronounced absorption at 1.92 .mu.m. The moisture content in a sample can therefore be measured using a back scattered radiation technique, i.e. by taking the ratio of the back scattered radiation at these two wavelengths to provide a direct measurement of the moisture content of the paper insulation.
In the present invention, an optical waveguide such as an optical fibre bundle is used to transmit radiation to and from the paper insulation. The end of the optical fibre is maintained close to, but not in contact with, the paper insulation, which prevents excessive absorption of the radiation by oil. This permits the moisture content to be determined directly in the paper insulation, on-site and without draining the transformer oil. Furthermore, by keeping the optical fibre bundle totally free of metal, the method and device of the present invention can be used in equipment on line, i.e. equipment energized to a high electric voltage, without any interruption in service.
This method of measuring moisture is most suitable and beneficial for absorbent materials that are not easily accessible. Also the method is non-destructive, in that no piece or sample is taken out of the absorbent material.